1. Technical Field of the Invention
The present invention relates to selectively adsorbing impurities contaminating gaseous flowstreams, particularly gas flows composed principally of hydrogen.
The present invention more especially relates to selective adsorption employing adsorbents obtained by exchanging the lithium atoms of zeolites of faujasite type and, even more especially, among these, zeolites having a low Si/Al ratio.
2. Description of the Prior Art
The purification of hydrogen via adsorption is a significant industrial process to date widely practiced. It entails the recovery of hydrogen from a mixture of several constituents originating from the catalytic reforming of natural gas, from plants for the production of ammonia or from ethylene synthesis units. The principle of pressure-modulated adsorption (hereinbelow PSA, for Pressure Swing Adsorption) is applied in order to obtain hydrogen of high purity. In practice, in order to properly operate a PSA process, the gas to be purified is conveyed through a bed of adsorbent at high pressure, during which stage the impurities are selectively adsorbed onto the adsorbent. When the latter is saturated, it is purged and then desorbed by lowering the pressure. The adsorbent thus being regenerated, recompression is carried out to return the system to the original starting conditions. This type of process, except for the thermal effects induced by the adsorption and the desorption of the molecules, is virtually isothermal and proves to be highly advantageous from an energy viewpoint.
The impurities are most typically CO2, NH3, N2, CO, CH4 and C1-C4 hydrocarbons, at contents ranging from a few ppm to a few percent. In practice, the adsorbent is a bed of alumina or silica gel for retaining water, of active charcoal for retaining CO2 and CH4 and of molecular sieve for trapping CO and N2.
The first industrial plant, which dates from 1967, is described by UCC in U.S. Pat. No. 3,430,418 and, to date, the zeolite adsorbent used is a molecular sieve of type 5A. Such zeolites have the faujasite structure, a description of which appears in Handbook of Molecular Sieves by R. Szostak, published by Van Norstrand Reinhold (1992). Zeolites are generally crystalline solids, the crystal lattice of which is formed by the condensation of tetrahedra, typically silicon and aluminum tetrahedra, by means of bridging oxygen atoms. In certain instances, the tetrahedra can contain other elements, such as boron, gallium, titanium, vanadium or iron, instead of aluminum, and essentially phosphorus in partial or complete substitution for silicon. It is also possible to prepare zeolites which contain only silicon oxide. These products are chemically analogous to silicas (quartz or cristobalite), but possess the principal characteristic of zeolites, namely, a perfectly calibrated and regular porosity. It is possible to adjust the proportion of silicon and of aluminum in a given structure: it is thus possible to prepare faujasite for Si/Al ratios of between 1 and 4. Depending on the types of zeolite, the pore sizes range from 0.3 to 0.8 nm (3 xc3x85 to 8 xc3x85) for solids formed by condensation of silicon and aluminum tetrahedra. Microporous structures of aluminophosphate type are known which exhibit pores of larger size, in the region of 1.2 nm (12 xc3x85). Certain zeolite structures possess only a single type of pore opening, whereas others exhibit two, indeed three, different pore networks intersecting or not within the crystal. These highly varied characteristics present numerous possibilities of using zeolites in fields as diverse as ion exchange, catalysis or adsorption.
The introduction of aluminum into the crystal lattice can be interpreted as a substitution of the tetravalent silicon by an element with a lower charge, in this case trivalent. As the number of oxygen atoms, and thus of negative charges, remains constant, a negative charge is established for each aluminum atom introduced. The charges are then balanced by the insertion of charge-compensating ions, such as alkali metals or alkaline earth metals. These cations are capable of being exchanged in solution, which contributes certain specific properties to the solid: thus, a zeolite A, in the sodium form, has a pore opening of 0.4 nm. When exchanged with calcium, the size of its pores increases slightly to 0.5 nm, which modifies the properties of the adsorbent. The latter then becomes capable of adsorbing normal paraffins, while excluding the bulkier branched paraffins. The exchange of zeolite A with potassium, on the other hand, reduces the pore size to 0.3 nm, which permits this zeolite to adsorb only water or ammonia.
The use of these adsorbents is thus potentially promising in the H2 PSA application. It is curious to observe that the literature is relatively sparse as regards useful adsorbents. Mitsubishi reports, in its Japanese Application JP-01080418, the zeolite of type X exchanged with calcium for adsorbing carbon monoxide. Studies relating to the influence of the electrostatic potential of the cation on the properties of adsorbing CO have been carried out by Bose et al., Proc. 6th Int. Zeolite Conf., p. 201 (1983). The authors have demonstrated that the decreasing direction of the energies for zeolite A was:
Mg less than Ca less than Li less than Na, 
which is the decreasing order of the charge/size ratios.
In certain systems, the authors recommend adsorbents capable of very selectively adsorbing CO, while minimizing the adsorption of N2 or CO2 (EP-A-224,150, Nippon Kokan Kabushiki Kaisha). This is accomplished by exchange and then impregnation, by means of a copper salt, of a zeolite of type Y or ZSM5: it is considered that the active species is monovalent copper, which is formed under reducing conditions and makes possible the formation of bonds between the xcfx80 electrons of the CO and an empty d orbital of the copper. Nevertheless, these adsorbents have the distinguishing feature of poorly regenerating under PSA conditions, due to the very high energies involved. The improvement of adsorption processes thus remains a serious need in this art, either to decrease the size of plants or to decrease operating costs.
It has now unexpectedly been determined that the lithium form of zeolites of faujasite type in which the Si/Al ratio of the lattice ranges from 1 to 3, preferably from 1 to 1.5, constitutes a class of adsorbents which is far superior to the adsorbents to date used for purifying hydrogen from the impurities which it contains, essentially carbon monoxide and nitrogen. These adsorbents have adsorption capacities for CO and N2 which are particularly high and present the advantage over adsorbents of faujasite type exchanged with calcium ions of exhibiting isotherms which are more linear, which is favorable for application in PSA.